Materials with nanoscale dimensions exhibit very unique mechanical, chemical, magnetic, electronic, and optical properties which are found neither at bulk scales nor at the molecular scale. These unique properties are often found to be highly dependent on the size of the nanoparticles, and as such, obtaining monodisperse samples of nanoparticles of a desired size is of the utmost importance for certain applications and fundamental studies. While it is possible to synthesize monodisperse nanoparticles, most notably through controlled growth methods (e.g. citrate reduction and seeded growth), these methods are typically tailored for a particular material and produce low-concentration, aqueous dispersions of nanoparticles which are challenging to process. Thus, post-processing methods for obtaining monodisperse fractions are required that will be applicable to many different types of materials and that offer higher throughputs. Current post-processing methods used to obtain monodisperse fractions from polydisperse samples include size-exclusion liquid chromatography, high-pressure liquid chromatography, isoelectric focusing electrophoresis, gel electrophoresis, and diafiltration. However, each of these methods require expensive and specialized equipment and provide relatively low throughput.
A typical technique used to size-selectively fractionate nanoparticles involves the use of a liquid solvent-antisolvent fractionation through an induced precipitation which requires centrifugation and can produce large quantities of organic waste. At application relevant processing scales (greater than milligram quantities), centrifugation is inefficient and expensive and large quantities of organic waste would be expensive to dispose of or require large amounts of energy to separate, thus making a liquid solvent-antisolvent a non-optimal solution.